Observations of a mysterious plume-like feature (marked with yellow arrow) at the limb of the Red Planet on March 20, 2012. The observation was made by astronomer W. Jaeschke. The image is shown with the north pole toward the bottom and the south pole to the top.

W. Jaeschke

Plumes seen reaching high above the surface of Mars are causing a stir among scientists studying the atmosphere on the Red Planet.

On two separate occasions in March and April 2012, amateur astronomers reported definite plume-like features developing on the planet.

The plumes were seen rising to altitudes of over 155 miles (250 kilometer) above the same region of Mars on both occasions. By comparison, similar features seen in the past have not exceeded 60 miles (100km).

The top image shows the location of the mysterious plume on Mars, identified within the yellow circle (top image, south is up), along with different views of the changing plume morphology taken March 21, 2012.

W. Jaeschke and D. Parker

“At about 250km [155 miles], the division between the atmosphere and outer space is very thin, so the reported plumes are extremely unexpected,” said Agustin Sanchez-Lavega of the Universidad del País Vasco in Spain.

The features developed in less than 10 hours, covering an area of up to 620 by 310 miles (1,000 by 500km), and remained visible for around 10 days, changing their structure from day to day.

None of the spacecraft orbiting Mars saw the features because of their viewing geometries and illumination conditions at the time.

However, checking archived Hubble Space Telescope images taken between 1995 and 1999 and of databases of amateur images spanning 2001 to 2014 revealed occasional clouds at the limb of Mars, albeit usually only up to 60 miles (100km) in altitude.

But one set of Hubble images from May 17, 1997, revealed an abnormally high plume, similar to that spotted by the amateur astronomers in 2012.

Scientists are now working on determining the nature and cause of the plumes by using the Hubble data in combination with the images taken by amateurs.

“One idea we’ve discussed is that the features are caused by a reflective cloud of water-ice, carbon dioxide-ice, or dust particles, but this would require exceptional deviations from standard atmospheric circulation models to explain cloud formations at such high altitudes,” said Agustin.

“Another idea is that they are related to an auroral emission, and indeed aurorae have been previously observed at these locations, linked to a known region on the surface where there is a large anomaly in the crustal magnetic field,” said Antonio Garcia Munoz from the European Space Agency’s ESTEC.

The jury is still out on the nature and genesis of these curious high-altitude martian plumes. Further insights should be possible following the arrival of ESA’s ExoMars Trace Gas Orbiter at the Red Planet, scheduled for launch in 2016.